Fixes silly mistake in build script.
Fixes silly mistake in build script.

"unlink" is part of "os", not "sys".

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MOS_MIGRATED_REVID=137975128

bazelbuild/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,bazelbuild/tulsi,pinterest/tulsi,pinterest/tulsi,pinterest/tulsi,pinterest/tulsi,pinterest/tulsi,pinterest/tulsi,bazelbuild/tulsi,bazelbuild/tulsi